Vanadium pentoxide is one of the important industrial vanadium products, and widely applied in the production of alloy additives such as ferrovanadium and vanadium nitride, and in the fields of catalysts, colorants, cemented carbide additives and the like. With the continuous development of new energy technologies, there is a growing demand on high-purity vanadium pentoxide (with a purity of above 3N5) in the battery industry, including an all-vanadium redox flow battery (VRB) with good large-scale energy storage performance, a vanadate-based lithium-ion battery used for electric automobiles and the like. However, in general, only vanadium pentoxide with a purity of 2N5 (i.e. the product according with the specification in HGT 3485-2003) can be prepared by the existing industrial technology, which is difficult to meet requirements on vanadium pentoxide for the battery industry. Therefore, how to prepare high-purity vanadium pentoxide with low cost and high efficiency is one of the urgent issues needed to be solved in the field of new energy technologies.
At present, high-purity vanadium pentoxide powder is usually obtained by the following method: a vanadium-leaching solution or a vanadium solution which is obtained by dissolving a vanadium-rich material (such as ammonium polyorthovanadate, ammonium metavanadate, industrial grade vanadium pentoxide, etc.) is used as a raw material, and purified by the method such as chemical precipitation purification and/or solvent extraction/ion resin exchange or the like, to obtain a purified vanadium solution; the purified vanadium solution is subjected to ammonium salt precipitation to obtain the purified ammonium polyorthovanadate or ammonium metavanadate precipitate; then, the precipitate is subjected to decomposition by calcination to obtain the high-purity vanadium pentoxide powder, as described in Chinese Patent Applications CN1843938A, CN102730757A, CN103145187A, CN103515642A, CN103194603A, CN103787414A, CN102181635A and CN103663557A, European Patent EP0713257B1, etc. In these methods, the process parameter for impurity removal is closely related to the content of the impurity in the raw material, thus the adaptability to the raw material is poor. Moreover, the calcium salt and magnesium salt scavengers or extractants, the acid and alkali reagents and ammonium salts for vanadium precipitation used in the purification process are also liable to introduce impurities. In order to improve the quality of the product, it is usually required to use expensive reagents with high purity, thereby leading to the following problems: the cost is too high, large-scale production cannot be implemented and the purity of the product is difficult to stabilize at above 3N5.
For the problems that the scavengers or extractants are liable to introduce impurities and the cost of the reagents used is too high, the relevant agencies also propose the use of the repeated precipitation method to achieve purification and impurity removal of a vanadium solution; that is, by using the ammonium salt precipitation characteristic of the vanadium-containing solution, vanadium is selectively precipitated out, to confine a part of the impurity ions to the solution after precipitation; the resulting ammonium salt precipitate is dissolved and then multiple repeated operations are conducted, to obtain more pure ammonium polyorthovanadate or ammonium metavanadate precipitate; and the precipitate is subjected to decomposition by calcination to obtain a high-purity vanadium pentoxide powder, as described in Chinese Patent Applications CN103606694A, CN102923775A, etc. This process effectively reduces the amount of the reagents used and the possibility that the reagents introduce impurities. However, the dissolution-precipitation process still requires use of a large quantity of high-purity acid and alkali reagents and ammonium salts, therefore the cost of purification is still high; and the cumbersome multiple precipitation operations not only lower the production efficiency but also lead to a significant decline in the direct recovery rate of vanadium. In addition, in the above-mentioned solution purification methods, extraction/back extraction, precipitation, washing and other operation steps will produce a large amount of waste water mainly containing a small quantity of vanadium ions and ammonium ions and a large amount of sodium salts, which results in difficult treatment and outstanding problem of pollution and also seriously restricts the large-scale industrial application of the methods.
Due to the large difference in the boiling points and saturated vapor pressures of metal chlorides, different metal chlorides are easily separated by distillation/rectification. Raw material chlorination—purification by rectification—subsequent treatment is a commonly-used preparation process for high-purity materials such as high-purity silicon (polysilicon), high-purity silicon dioxide, and the like. Because of a very large difference between boiling points of the chloride of vanadium, vanadium oxytrichloride, and the chlorides of common impurities such as iron, calcium, magnesium, aluminum, sodium, potassium and the like, high-purity vanadium oxytrichloride is easily obtained by rectification, and high-purity vanadium pentoxide can be prepared by subjecting the high-purity vanadium oxytrichloride to hydrolysis and ammonium salt precipitation, supplemented by calcination. Therefore, the use of the chlorination method for the preparation of high-purity vanadium pentoxide has a greater advantage in principle. In fact, the use of the chlorination method for the preparation of high-purity vanadium pentoxide is not only feasible in principle, but also has been implemented in the laboratory by the researchers of Iowa State University in the United States as early as the 1960s (Journal of the Less-Common Metals, 1960, 2: 29-35). They employed ammonium polyorthovanadate as a raw material, and prepared the crude vanadium oxytrichloride by chlorination with addition of carbon, then obtained high-purity vanadium oxytrichloride through purification by distillation, and conducted ammonium salt precipitation to obtain high-purity ammonium metavanadate, and finally calcined high-purity ammonium metavanadate at 500-600° C., to obtain the high-purity vanadium pentoxide powder. However, a large amount of wastewater containing ammonia and nitrogen will be produced in the precipitation and the washing processes (at least 1.8 ton of ammonium chloride waste salt is produced per ton of a vanadium pentoxide product), leading to difficult treatment; and the precipitation, drying and calcination processes of ammonium salts not only require high energy consumption, but also easily cause environmental pollution. In addition, the study only realizes the intermittent preparation of high-purity vanadium pentoxide by the chlorination method with the laboratory equipment, and cannot provide related information on how to use the chlorination method for continuous preparation of high-purity vanadium pentoxide on an industrial scale. It may be for exactly these reasons that the report on continuous preparation of high-purity vanadium pentoxide by the chlorination method is difficult to find in the decades after the study.
Recently, Chinese Patent Application CN103130279A proposes a method for preparing high-purity vanadium pentoxide by using the chlorination method with a vanadium-iron magnetic iron ore, vanadium slag, vanadium-containing catalyst and other materials containing vanadium as raw materials. A mixture of chlorides of vanadium is obtained through chlorination with addition of carbon—dust removal—condensing, and vanadium tetrachloride is separated through rectification to obtain pure vanadium oxytrichloride, then the vanadium oxytrichloride is fed into an ultrapure aqueous solution or ultrapure aqueous solution of ammonia and precipitated, and the precipitate is filtered, dried and calcined to obtain vanadium pentoxide. This patent has the following deficiencies: (1) similar to the above study of Iowa State University in the United States, this patent actually provides the basic flow of chlorination only, lacking the specific operable solutions. For example, the method of chlorination comprises both boiling chlorination and molten salt chlorination, which are completely different methods of chlorination. For another example, concerning the chlorination reactor, it is proposed to use reactors such as “rotary kiln, fluidized furnace, boiling furnace, shaft furnace, multi-hearth furnace” and the like, which actually covers almost all of the commonly-used mainstream reactors in the metallurgical industry; however, different reactors' requirements for raw materials differ greatly. For example, the shaft furnace can only handle “coarse” particles with a particle size more than 8 mm, and needs to conduct pelleting and sintering pretreatment when “fine” particles are processed, while boiling chlorination is generally suitable for the treatment of fine particles. Therefore, a particular vanadium raw material cannot be directly applied to rotary kiln, fluidized furnace, boiling furnace, shaft furnace, multi-hearth furnace and other reactors. Moreover, the “fluidized furnace” and “boiling furnace” are essentially the same, just different in names; therefore, since these reactors vary widely in operation mode and condition, the method cannot actually be implemented on the condition that only basic flow is provided. (2) Vanadium oxytrichloride is fed into the ultrapure aqueous solution for hydrolysis. However, because vanadium pentoxide is easily dissolved in the hydrochloric acid solution, the recovery rate of precipitation of vanadium is too low. Moreover, in the hydrochloric acid solution with an HCl concentration more than 6.0 mol/L, when vanadium pentoxide is dissolved, it will be reduced to VOCl2 and chlorine gas is released, which will further reduce the recovery rate of precipitation of vanadium. Precipitation and washing processes will inevitably produce a large amount of hydrochloric acid solution containing vanadium, and it is difficult to effectively achieve a comprehensive treatment.
In addition, for large-scale industrial applications, there still exists the following two problems in the existing technologies for chlorination of vanadium raw materials: (1) calcination for chlorination of vanadium raw materials is a strong exothermic process, and in addition to preheating the solid and gas reaction materials, the heat generated by the chlorination reaction still needs to be removed by furnace wall heat dissipation to stabilize the temperature in the chlorination; therefore, both the solid and gas are usually enters the reactor at a temperature of near room temperature, and only can participate in the reaction after been preheated by the heat produced from the chlorination reaction, resulting in too low efficiency of reaction in part of the chlorination reactor; (2) since the heat produced by the chlorination reaction needs to be removed through dissipation of a large amount of heat to maintain the operation temperature, the operating condition and environmental climate change are both liable to cause fluctuations in chlorination temperature, resulting in reduction of selectivity in chlorination and efficiency, and it is needed to use a reasonable method for balanced supply of heat and temperature regulation. Therefore, reasonable heat supply and temperature control must be provided. Only in this way, it is possible to effectively improve the efficiency of chlorination and obtain stable chlorination temperature, so as to ensure the selectivity in the chlorination to effectively inhibit the chlorination of impurities.
Therefore, achieving the temperature regulation of chlorination process, improving the direct recovery rate of vanadium, reducing the amount of the waste discharged, and decreasing the energy consumption and chlorine consumption in production, are the keys to increase the economy of the technology for purifying and preparing high-purity vanadium pentoxide through the chlorination method.